To appear as Chapter 2 in "Supermassive Black Holes in the Distant Universe" (2004), ed. A. J. Barger, Kluwer Academic Publishers, in press
astro-ph/0405144

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Abstract. We discuss the very different methods in each wavelength band for selecting and finding Active Galactic Nuclei (AGN). We briefly review the history of the different techniques for finding AGN and compare and contrast the advantages and difficulties of selection in different wavelength bands. We stress the strong selection effects in each wavelength band and the difficulty of defining complete samples. Of all the techniques presently used, we conclude that selection in the hard X-ray band via imaging and spectroscopy is the most complete and allows the best estimate of the number and evolution of active galaxies. However, all of the techniques have difficulties at low luminosities where emission due to stellar processes can have similar sizes and luminosities.

Table of Contents

INTRODUCTION

A Short History of AGN Search Techniques

AGN SPECTRAL ENERGY DISTRIBUTION AND HOW ONE FINDS AGN

Optical Color Selection and the Presence of a Semi-stellar Nucleus

Optical Emission Lines, Variability, and Zero Proper Motion Selection

Radio Selection

Infrared Selection

High-Energy Selection

Ultraviolet Selection

SELECTION EFFECTS

X-RAY SELECTION OF AGN

Early X-ray Surveys

Soft X-ray Surveys

Pre-Chandra and XMM-Newton Hard X-ray Surveys

Deep Chandra and XMM-Newton Hard X-ray Surveys

Comparison of X-ray and Optical Surveys

Very High Energies

CONCLUSIONS

REFERENCES